The conventional mechanical seal used between a motor and a product pump comprises a stationary seal ring connected to the motor housing and a rotatable seal ring connected to the motor shaft, each seal ring having a lapped seal face opposing the seal face on the other ring. Resilient means such as coil springs and/or bellows urge one seal ring face toward the other in sealing relation.
In order to facilitate the installation and removal of such seals, it is common to assemble the seal in a so-called "cartridge," such as by mounting a pre-assembled seal on a cylindrical sleeve which is slipped on the O.D. of the motor shaft and secured thereto by one or more set-screws.
Most mechanical seals of the type described above have a number of metal parts, such as bellows and springs, which bias the seal rings in sealing relation, and are exposed to the product fluid. In applications where the product fluid is non-abrading and non-corrosive, this does not present a problem; but in slurry pumps, the abrasive action of the pump fluid attacks the metal parts and greatly reduces service life.
While the typical mechanical seal is compact and is charcterized by closely fitting, precision made parts, it is desirable to provide slurry pump seals with rather crude parts which can be manufactured with larger dimensional tolerances and therefore at lower cost, because they wear out more quickly despite efforts to extend operating life. This also affects the manner in which they have to be mounted in the pump. For instance, as mentioned above, most cartridge seals are secured with a number of set screws which often become frozen and require a tedious, time-consuming operation to free up. In dealing with slurry pump seals, which are adjusted and replaced more often, it is important to provide a quick and uncomplicated means for securing and removing the seal from the shaft.
Still another consideration in the design of slurry pumps seals is in the area of loading the seal rings. There are examples of seals in the prior art where the springs have been replaced by an elastomeric material to provide the necessary "spring" to keep the seal rings in sealing relation.
Elastomers can be stressed in torsion, compression, tension and shear. The latter is the most advantageous because the stresses are spread, generally uniformly, throughout the mass of the elastomers. When stressed in torsion, the stresses are primarily in the outer fibers with the interior generally unstressed and it is these fibers which are the first to be lost through abrasion or chemical attack. Further, an elastomer stressed in torsion quickly loses its spring function. When placed in tension, elastomers take on the permanent set of the stressed material, as for example, a stretched rubber band becomes permanently elongated. If an elastomer body is confined, it is incompressible. If not confined and subject to compression, it will bulge; moreover, it will have a high spring rate with little travel, so it is not suitable for substitution as a spring.
For example, Deane et al, in U.S. Pat. No. 4,306,727, teach the use of a static elastomeric ring abutting a metal seal ring in a drill bit environment and discloses, in the background, that elastomeric seals are used between the rotating cutters and the bearing journals to prevent intrusion of dirt, sand, rock cuttings, corrosive liquids and other contaminants into the bearing area. Although the elastomeric ring of Deane et al does show the use of an elastomer to urge one seal ring toward one another, it is constructed in such a manner that debris tends to collect on the leakage side which could cause deterioration of the elastomer and be difficult to clean out except by disassembly of the seal mechanism.
In Voitek, U.S. Pat. No. 3,272,519, seals for use between the rear wheels of a tractor and a tractor frame are described. Each uses a pair of elastomeric, "rounded square" cross-sectioned, sealing elements which urge a pair of seal rings toward one another and which, during use, are stressed and deform to a diamond shape, leaving crevices at the concave grooves into which the elastomeric members are received to collect debris.
Neither Deane et al nor Voitek disclose or suggest the concept of loading the elastomeric element in shear.